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Question DetailsAsked on 11/29/2016

My husband unwired thermostat to install new paneling. He rewired. Thermostat showed 75.It went to 84 on cool.WHY?

The temp showed 75 and i turned it down to 74. Later I noticed room seemed hotter. That's when I saw that temp was 84 on COOL. Would COOL setting blow hot air if he miswired it when hooking it back up? It was fine before.

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2 Answers


Simplest explanation - he did not turn the thermostat OFF before disconnecting wires or removing batteries, so the system got confused - or he disconnected batteries (if it is electronic) or external power (if powered off HVAC appliances) and it lost its memory settings. In either case, turning to OFF, removing batteries for 1 minute or longer, then putting back in and turning to the designated setting in the manual (may be OFF to program, or may be both HEAT and A/C to program those two functions, depending on brand) and reprogramming may be all that is needed.



There are various wiring configurations for thermostats - hopefully your wires are color-coded to they can be easily traced to the device they serve and to the specific connection points at each end. Commonly, heating and A/C combined thermostats have 3 or more commonly 4 wires at the thermostat - one live control or "demand" feed outbound each to A/C and furnace; and either a common power feed (in an intergrated system) or a power feed from each device - commonly but not always 24V. Occasionally there is also a neutral feed - common or from each device, so that can mean a total of from 3 up to 6 wires coming into and out of the device, total. Add a whole house fan or recirculator and you can be up to 8 wires total.

Generally - but there are exceptions - a "live" wire comes from the appliance to the thermostat, which then returns a live "control" or "demand" feed from the thermostat to the controller at the device. This return feed is "live" or "interrupted" depending on whether the thermostat is calling for service from that device at a given time. in the simplest case with manual or battery-operated electronic thermostats (so the thermostat needs no power feed to it), only the "live" wire from and back to the appliance is run through the thermostat - the neutral does not come to it at all.

A miswiring that resultad in no signal going to the A/C could mean the signal never got out of the thermostat - meaning the house temperature would just move towards equalizing with the outside temperature, without any fan operation unless a whole-house fan or continuous HVAC fan operation setup existed.

If the forced air system was running (and it does not usually do so as a whole-house continuous air recirculation system), and was blowing hot air on the A/C setting, then most likely the furnace wire got hooked to the A/C connection, and presumably vice-versa. Swapping the A/C and furnace wires, for instance, could have one of several effects - worst would be if they were different voltage systems, though that is rare, so would short out the controls and possibly damage the thermostat. Swapping the incoming power wires, if same voltage, would in normal operations probably have no effect other than that you would have cross-circuited two devices, meaning potentially when the power is supposed to be OFF to one device it could be cross-fed by the other and still be on. Also, if you powered a device down seasonally and it was miswired, then the other device (which thermostat function was actually pulling power from that first one) would no longer operate. Ditto if the "feeding" device shut itself down due to a safety device or overload or such.

Mis-matching the outbound "demand" or "control" feeds to the HVAC appliance controllers would mean when the A/C is supposed to be on the furnace actually would be activated, and vice versa. Sounds like this might be what happened in your case - so when set for A/C operation and you turned the thermostat down to 74 it applied power to the A/C outlet from the thermostat to cool the house by turning the A/C on - but that connection was actually wired to the furnace so it turned the furnace on - and of course because it kept getting hotter with the furnace on the thermnostat continued to call for continued A/C operation, but was actually sending the signal to the furnace to operate - so it would heat up till the house reached a stable hot condition where the furnace was operating continuously at full power and the heat going into the house eventually equaled the losses through the walls and roof and such - which might be a good 20-50 degrees or more above a warm outside temperature in a typical house, until the furnace shut down from overheating.

Generally speaking - but only generally, not universally, if there are two pairs of wires coming to the thermostat (two separate wire runs, 2 wires on each insulated bundle) one pair would be power and demand wires for the A/C, on for the furnace - the power one should be red or black and the other commonly white, but not always. Also, while many thermostats (manual ones or ones with battery electronic control) might work with the power and demand wires from a given device swapped, that could leave the housing of the thermostat "live" and create a hazard or the opportunity for the thermostat to be fried - so just randomly testing different wires different places is NOT the way to go.

It MIGHT be that swapping the two outbound thermostat leads between A/C and Heat MIGHT solve the problem - might also short out the thermostat and potentially the controllers, so the right thing to do is find the wiring diagrams for the A/C and furnace and thermostat (usually in owner's manual that came with the appliance, or on manufacturer website, or at SearsAtHome and other websites in many cases) or see if labelled at the device and thermostat (almost always labelled on thermostat) and track down the leads - which are power to the thermostat, which are control from the thermostat, and which are neutral if used; from/to each device - and be sure (hopefully with color-coded wires, otherwise by physically or electrically or radio tracer tracking the wires) that each is hooked up where in appliance control board which might cost up to about $500-600 to replace - up to $2000 in an integrated home climate control system because they can have a mixture of 12, 24, 48, and 120V leads - and I have seen some with DC controls as well, which might result in fried wiring in the walls as well. Note also that furnace unit/zone heaters and some in-floor systems also commonly have 120V thermostats controlling them, so you can have both low voltage and household voltage power coming off the devices to thermostats - you do NOT want to miswire those.

Professionally - a Heating and A/C contractor is the Search the List category for a tech to correct this.

Word to the wise for next time panelling or painting - mark the wires and the connections - use colored marker or a number to mark each connection on the thermostat, on the wires color code with permanent colored marker or heat shrink or mark the corresponding connection number for the conenction with that number of bands around the insulation on the wire with a black indelible marker. Or tie/tape on string label tags, making sure they cannot slide or easily be pulled off- this is use #37 for electrical tape or duct tape.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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